Researchers have long been puzzled by the phenomenon of static electricity generated by rubbing objects together.
After more than two millennia, scientists at Northwestern University have finally unraveled the mechanics behind this process. They discovered that when an object slides, different forces act on its front and back parts, leading to the buildup of different electrical charges. This difference in charges creates a current, resulting in a small but noticeable zap.
“For the first time, we are able to explain a mystery that nobody could before why rubbing matters,” said Northwestern’s Laurence Marks, who led the study. “People have tried, but they could not explain experimental results without making assumptions that were not justified or justifiable. We now can, and the answer is surprisingly simple. Just having different deformations — and therefore different charges — at the front and back of something sliding leads to current.”
With expertise in surface structures, Marks serves as a professor emeritus of materials science and engineering at Northwestern’s McCormick School of Engineering. The paper’s primary author is Karl Olson, a Ph.D. student in Marks’ research group.
The discovery of friction-induced static electricity was first documented by Greek philosopher Thales of Miletus in 600 B.C. He observed that after rubbing amber with fur, the fur attracted dust.
“Since then, it has become clear that rubbing induces static charging in all insulators — not just fur,” Marks said. “However, this is more or less where the scientific consensus ended.”
In 2019, Marks and his team embarked on a journey to unravel a scientific mystery. Their groundbreaking study, published in Physical Review Letters, revealed that the act of rubbing two materials together triggers the bending of minuscule protrusions on their surfaces, generating voltages.
Their initial insights have now come to fruition, leading to the development of a new model for calculating electrical current. At the heart of this model lies the concept of “elastic shear,” which explains how materials resist sliding forces, leading to the generation of electrical charges.
Their findings have significant implications for understanding and harnessing the relationship between mechanical actions and electrical currents.
“Sliding and shear are intimately connected,” Marks said.
While static electricity may result in comical incidents, such as hair standing up after sliding down a playground slide, it also poses serious risks. Sparks from static electricity can lead to industrial fires and explosions, as well as disrupt the consistent dosing of powdered pharmaceuticals. By gaining a deeper understanding of these mechanisms, researchers have the potential to develop innovative solutions to address these pressing issues.
“Static electricity affects life in both simple and profound ways,” Marks said. “Charging grains with static electricity has a major influence on how coffee beans are ground and taste. The Earth would probably not be a planet without a key step in the clumping of particles that form planets, which occurs because of the static electricity generated by colliding grains. It’s amazing how much of our lives are touched by static electricity and how much of the universe depends on it.”
Journal reference:
- Karl P. Olson and Laurence D. Marks. What Puts the “Tribo” in Triboelectricity? Nano Letters, 2024; DOI: 10.1021/acs.nanolett.4c03656